One of America's most popular whisky makers admits it is watering down its bourbon to keep up with soaring global demand.

Maker's Mark told its most loyal customers in a letter recently that the company has altered its recipe in a way that will reduce the bourbon's proof from 90 to 84 in order to produce enough bourbon to meet orders.

That will reduce the alcohol content of the company's signature product from 45 per cent by volume, to 42 — reduction of almost seven per cent.

"Fact is, demand for our bourbon is exceeding our ability to make it, which means we're running very low on supply," brothers Rob and Bill Samuels said. They are the two executives who head up the family-run company.

Maker's Mark blames unforeseen issues in its supply chain for the change. Typically, the company sells bourbon that has been aged between five years nine months and seven years before bottling it. But rising international demand for the comparatively sweet American liquor has made it impossible for the company to stay within that range.

The company insists customers won't notice a difference in taste, and says it is working hard to upgrade its production capacity and expand its distillery.

"All bourbons are cut with water to achieve the desired proof for bottling," Maker's Mark chairman Bill Samuels said. "This is a natural step in the bourbon-making process."

The company isn't the first to water down its product in the face of a surge in demand. Ubiquitous Tennessee whisky Jack Daniel's lowered its proof to 80 from 86 in 2004, reducing the alcohol content from 43 to 40 per cent in the process.